The next generation of wireless communication systems is required to provide high quality voice services as well as broadband data services with data rates far beyond the limitations of current wireless systems. For example, high speed downlink packet access (HSDPA), which is endorsed by the 3rd generation partnership project (3GPP) standard for wideband code-division multiple access (WCDMA) systems, is intended to provide data rates up to 10 Mbps or higher in the downlink channel as opposed to the maximum 384 Kbps supported by the enhanced data rate for GSM evolution (EDGE), the so-called 2.5G communication standard, see 3GPP: 3GPP TR25.848 v4.0.0, “3GPP technical report: Physical layer aspects of ultra high speed downlink packet access,” March 2001, and ETSI. GSM 05.05, “Radio transmission and reception,” ETSI EN 300 910 V8.5.1, November 2000.
Antenna diversity can increase the data rate. Antenna diversity effectively combats adverse effects of multipath fading in channels by providing multiple replicas of the transmitted signal at the receiver. Due to the limited size and cost of a typical end user device, e.g., a cellular telephone or handheld computer, downlink transmissions favor transmit diversity over receiver diversity.
One of the most common transmit diversity techniques is space-time coding, see Alamouti, “A simple transmit diversity technique for wireless communications,” IEEE J Select. Area Commun., vol. 16, pp. 1451–1458, October 1998, Tarokh et al., “Space-time codes for high data rate wireless communication: performance criterion and code construction,” IEEE Trans. Info. Theory, vol. 44, pp. 744–765, March 1998, Tarokh wt al., “Space-time block codes from orthogonal designs,” IEEE Trans. Info. Theory, vol. 45, pp. 1456–1467, July 1999, and Xin et al., “Space-time diversity systems based on linear constellation preceding,” IEEE Trans. Wireless Commun., vol. 2, pp. 294–309, March 2003.
With space-time coding, data symbols are encoded in both the time domain (transmission intervals) and the space domain (transmit antenna array). For systems with exactly two transmit antennas, Alamouti et al. describes orthogonal space-time block code (STBC). Full diversity order is achieved with simple algebraic operations.
Space-time trellis coding exploits the full potential of multiple antennas by striving to maximize both the diversity gains and coding gains of the system. Better performance is achieved at the cost of relative higher encoding and decoding complexity.
The above techniques are designed under the assumption that the transmitter has no knowledge of the fading channels. Thus, those techniques can be classified as having open loop transmit diversity.
System performance can be further improved when some channel information is available at the transmitter from feedback information from the receiver. Those systems are classified as having closed loop transmit diversity. The feedback information can be utilized in transmit diversity systems to maximize the gain in the receiver, see Jongren et al., “Combining beamforming and orthogonal space-time block coding,” IEEE Trans. Info. Theory, vol. 48, pp. 611–627, March 2002, Zhou et al., “Optimal transmitter eigen-beamforming and space-time block coding based on channel mean feedback,” IEEE Trans. Signal Processing, vol. 50, pp. 2599–2613, October 2002, Rohani et al., “A comparison of base station transmit diversity methods for third generation cellular standards,” Porc. IEEE Veh. Techno. Conf. VTC'99 Spring, pp. 351–355, May 1999, Derryberry et al., “Transmit diversity in 3G CDMA systems,” IEEE Commun. Mag., vol. 40, pp. 68–75, April 2002, Lo, “Maximum ratio transmission,” IEEE Trans. Commun., vol. 47, pp. 1458–1461, October 1999, Huawe, “STTD with adaptive transmitted power allocation,” TSGR1-02-0711, May, 2002, and Homg et al., “Adaptive space-time transmit diversity for MIMO systems,” Proc. IEEE Veh. Techno. Conf. VTC'03 Spring, pp. 1070–1073, April 2003.
The space-time block coding can be combined with linear optimum beamforming. Linear encoding matrices can be optimized based on the feedback information of the fading channels. Transmit adaptive array (TxAA) is another closed loop transmit diversity system with the transmitted symbols encoded only in the space domain. Increased performance can be achieved, provided the fading channel vector is known to the transmitter. The concept of space encoded transmit diversity can be generalized as maximal ratio transmission (MRT).
All of the above closed loop systems require the feedback information to be M×N complex-valued vectors, where M and N are respectively the number of antennas at the transmitter and receiver. The vector elements are either the channel impulse response (CIR), or statistics of the CIR, e.g., mean or covariance. Considerable bandwidth is consumed by the feedback information in the reverse link from the receiver to the transmitter.
To overcome this problem, suboptimum methods with less feedback information are possible. Adaptive space-time block coding (ASTTD) uses a real-valued vector made up of power ratios of the fading channels as feedback information. There, the feedback information is used to adjust the power of each transmission antenna. That technique still consumes a large number of bits.
Therefore, it is desired to maximize transmit diversity gain while reducing the number of bits that are fed back to the transmitter.